Causes of Hair Loss Explained

Causes of hair loss: the 8 most frequent

Hair loss rarely has a single cause. In clinical practice, we almost always find one main factor combined with two or three aggravating ones — and it's this combination that treatment needs to address.

Clinically reviewed by Dr Fátima Garcês, Clinical Director of Replace Clinic — 20 years dedicated to hair restoration.

1. Genetics and hormones (androgenetic alopecia)

The most common cause. Genetically sensitive follicles miniaturise cycle after cycle in response to DHT. It's recognised by its pattern: temples and crown in men, a widening parting in women.

2. Iron deficiency and low ferritin

The follicle is one of the body's structures with the highest cell division rate, and one of the first to suffer when iron is low. Ferritin levels below 30 to 40 ng/mL are often associated with diffuse shedding, even without declared anaemia, although the evidence isn't entirely consistent — which is why it's important to interpret the result in clinical context.

3. Thyroid dysfunction

Both hypothyroidism and hyperthyroidism disrupt the hair cycle and cause diffuse shedding, often accompanied by a change in hair texture. Once thyroid function is corrected, hair recovers over several months.

4. Stress, surgery, fever and childbirth

Any physiological or emotional shock can push a large proportion of follicles into the resting phase at the same time. Shedding appears two to three months after the event — which is why the cause and effect are hard to connect. It's reversible in most cases.

5. Medication

Anticoagulants, retinoids, anticonvulsants, some antidepressants, beta-blockers and chemotherapy are among the drugs associated with hair loss. Stopping or switching medication should always be discussed with the prescribing doctor.

6. Restrictive diets and rapid weight loss

Deficiencies in protein, zinc, iron and calories compromise hair growth (biotin deficiency, though heavily marketed, is in practice rare). Weight loss of more than 10% within a few months frequently triggers telogen effluvium.

7. Traction and mechanical damage

Tight ponytails, extensions, braids and repeated use of heat and chemicals damage the follicle. If sustained for years, traction alopecia becomes scarring and irreversible.

8. Scalp conditions

Seborrhoeic dermatitis, psoriasis, lichen planopilaris and folliculitis cause inflammation that interferes with the follicle. Scarring alopecias require prompt diagnosis, because the follicular loss they cause is permanent.

Frequently asked questions

How do I know which cause applies to me?

By the pattern of shedding, clinical history and blood tests. Trichoscopy distinguishes androgenetic miniaturisation from diffuse shedding and from signs of scarring alopecia.

Can low iron cause hair loss even without anaemia?

Yes. Ferritin can be low with normal haemoglobin and still compromise the hair cycle.

How long after stress does hair start falling out?

Typically 2 to 3 months after the event, because follicles go through a resting phase before releasing the hair.

If I correct the cause, will my hair come back?

For reversible causes, yes — recovery usually takes between 6 and 12 months. In androgenetic alopecia, progression is halted, but it requires ongoing treatment.